Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 299,446	$ 237,851
Gross Unrealized Gains	37	0
Gross Unrealized Losses	(289)	(79)
Fair Value	299,194	237,772
Treasury securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		149,996
Gross Unrealized Gains		0
Gross Unrealized Losses		(1)
Fair Value		149,995
Commercial paper
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	93,452	37,963
Gross Unrealized Gains	8	0
Gross Unrealized Losses	(7)	(15)
Fair Value	93,453	37,948
State and municipal bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	17,798
Gross Unrealized Gains	5
Gross Unrealized Losses	(4)
Fair Value	17,799
Corporate bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	188,196	49,892
Gross Unrealized Gains	24	0
Gross Unrealized Losses	(278)	(63)
Fair Value	$ 187,942	$ 49,829